1. Field of the Invention
This invention relates to electron beam sources and, more particularly, to photocathodes for the generation of single or multiple electron beams.
2. Prior Art
High resolution electron beam sources are used in systems such as scanning electron microscopes, defect detection instruments, VLSI testing equipment, and electron beam (e-beam) lithography. In general, e-beam systems include an electron beam source and electron optics. The electrons are accelerated from the source and focused to define an image at a target. These systems typically utilize a physically small electron source having a high brightness.
Improvements in optical lithography techniques in recent years have enabled a considerable decrease in the linewidths of circuit elements in integrated circuits. Optical methods, however, will soon reach their resolution limits. Production of smaller linewidth circuit elements (i.e., those less than about 0.1 xcexcm) will require new techniques such as X-ray or e-beam lithography.
In e-beam lithography, a controllable source of electrons is desired. A photocathode used to produce an array of patterned e-beams is shown in FIG. 1. U.S. Pat. No. 5,684,360 to Baum et al., xe2x80x9cElectron Sources Utilizing Negative Electron Affinity Photocathodes with Ultra-Small Emission Areas,xe2x80x9d herein incorporated by reference in its entirety, describes a patterned photocathode system of this type.
FIG. 1 shows a photocathode array 100 with three photocathodes 110 comprising a transparent substrate 101 and a photoemission layer 102. The photocathode is back-illuminated with light beams 103 which are focused on photoemission layer 102 at irradiation region 105. As a result of the back-illumination onto photoemission layer 102, electron beams 104 are generated at an emission region 108 opposite each irradiation region 105. Other systems have been designed where the photoemitter is front-illuminated, i.e. the light beam is incident on the same side of the photoemitter from which the electron beam is emitted.
Often, light beams 103 or electron beams 104 are masked. In FIG. 1, light beams 103 are masked using mask 106 which allows light onto irradiation spots 108 but prevents light from being incident on other areas of photoemission layer 102. FIG. 1 also shows mask 107 which allows electrons to exit photoemission layer 102 only at certain surface spots corresponding to emission regions 105. A photocathode may also have a mask between transparent substrate 101 and photoemission layer 102 to block light beam 103 so that it is only incident at irradiation spots 105. In general, photocathode 110 may include no masking layers or may have one or more masking layers.
Each irradiation region 105 may be a single circular spot representing a pixel of a larger shape, the larger shape being formed by the conglomerate of a large number of photocathodes 110 in photocathode array 100. In that case, irradiation region 105 may be as small as is possible given the wavelength of the light beam incident on photocathode 100. Typically, a grouping of pixel irradiation regions has dimensions of 100-200 xcexcm. Each pixel can have dimensions (i.e. diameter) as low as 0.1 xcexcm. Alternatively, irradiation spot 105 and emission region 108 can be a larger shape. In either case, the image formed by emission region 108 will be transferred to e-beam 104 so long as the entirety of irradiation region 105 is illuminated by light beam 103.
Photoemission layer 102 is made from any material that emits electrons when irradiated with light. These materials include metallic films (gold, aluminum, etc.) and, in the case of negative affinity (NEA) photocathodes, semiconductor materials (especially III-V compounds such as gallium arsenide). Photoemission layers in negative electron affinity photocathodes are discussed in Baum (U.S. Pat. No. 5,684,360).
When irradiated with photons having energy greater than the work function of the material, photoemission layer 102 emits electrons. Typically, photoemission layer 102 is grounded so that electrons are replenished. Photoemission layer 102 may also be shaped at emission region 108 in order to provide better irradiation control of the beam of electrons emitted from emission region 108. Further control of the e-beam is provided in an evacuated column as shown in FIG. 2.
Light beams 103 usually originate at a laser but may also originate at a lamp such as a UV lamp. The laser or lamp output is typically split into several beams in order to illuminate each of focal points 105. A set of parallel light beams 103 can be created using a single laser and a beam splitter. The parallel light beams may also originate at a single UV source. Alternatively, the entire photoemission array 100 may be illuminated if the light source has sufficient intensity.
Photons in light beam 103 have an energy of at least the work function of photoemission layer 102. The intensity of light beam 103 relates to the number of electrons generated at focal point 105 and is therefore related to the number of electrons emitted from emission region 108. Photoemission layer 102 is thin enough and the energy of the photons in light beam 103 is great enough that a significant number of electrons generated at irradiation region 103 migrate and are ultimately emitted from emission layer 108.
Transparent substrate 101 is transparent to the light beam and structurally sound enough to support the photocathode device within an electron beam column which may be a conventional column or a microcolumn. Transparent substrate 101 may also be shaped at the surface where light beams 103 are incident in order to provide focusing lenses for light beams 103. Typically, transparent substrate 101 is a glass although other substrate materials such as sapphire or fused silica are also used.
If mask 106 is present either on the surface of transparent substrate 101 or deposited between transparent substrate 101 and photoemission layer 102, it is opaque to light beam 103. If mask 107 is present, it absorbs electrons thereby preventing their release from emission region 108. Mask 107 may further provide an electrical ground for photoemission layer 102 provided that mask 107 is conducting.
Photocathode 100 may be incorporated within a conventional electron beam column or a microcolumn. Information relating to the workings of a microcolumn, in general, is given in the following articles and patents: xe2x80x9cExperimental Evaluation of a 20xc3x9720 mm Footprint Microcolumn,xe2x80x9d by E. Kratschmer et al., Journal of Vacuum Science Technology Bulletin 14(6), pp. 3792-96, Nov./Dec. 1996; xe2x80x9cElectron Beam Technology xe2x80x94SEM to Microcolumn,xe2x80x9d by T. H. P. Chang et al., Microelectronic Engineering 32, pp. 113-130, 1996; xe2x80x9cElectron Beam Microcolumn Technology And Applications,xe2x80x9d by T. H. P. Chang et al., Electron-Beam Sources and Charged-Particle Optics, SPIE Vol. 2522, pp. 4-12, 1995; xe2x80x9cLens and Deflector Design for Microcolumns,xe2x80x9d by M. G. R. Thomson and T. H. P. Chang, Journal of Vacuum Science Technology Bulletin 13(6), pp. 2445-49, November/December 1995; xe2x80x9cMiniature Schottky Electron Source,xe2x80x9d by H. S. Kim et al., Journal of Vacuum Science Technology Bulletin 13(6), pp. 2468-72, November/December 1995; U.S. Pat. No. 5,122,663 to Chang et al.; and U.S. Pat. No. 5,155,412 to Chang et al., all of which are incorporated herein by reference.
FIG. 2 shows a typical electron beam column 200 using photocathode array 100 as an electron source. Column 200 is enclosed within an evacuated column chamber (not shown). Photocathode array 100 may be completely closed within the evacuated column chamber or transparent substrate 101 (FIG. 1) may form a window to the vacuum chamber through which light beams 103 (FIG. 1) gain access from outside the vacuum chamber. Electron beams 104 (FIG. 1) are emitted from emission region 108 (FIG. 1) into the evacuated column chamber and carry an image of emission region 108 (FIG. 1). Electron beam 104 may be further shaped by other components of column 200.
Electron beams 104 are accelerated between photocathode array 100 and anode 201 by a voltage supplied between anode 201 and photoemission layer 102. The voltage between photocathode array 100 and anode 201, created by power supply 208 (housed outside of the vacuum chamber), is typically a few kilovolts to a few tens of kilovolts. The electron beam then passes through electron lens 204 that focuses the electron beam onto limiting aperture 202. Limiting aperture 202 blocks those components of the electron beams that have a larger emission solid angle than desired. Electron lens 205 refocuses the electron beam. Electronic lenses 204 and 205 focus and demagnify the image carried by the electron beam onto target 207. Deflector 203 causes the electron beam to laterally shift, allowing control over the location of the image carried by the electron beam on a target 207.
In 0.1 xcexcm lithography systems, the size of a circular pixel incident on target 207 is on the order of 0.05 xcexcm. Therefore, the image of emission region 108 (FIG. 1) needs to be reduced by roughly a factor of 2 to 10, depending on the size of emission region 108 (FIG. 1). Target 207 may be a semiconductor wafer or a mask blank.
Conventional variable shaped electron beam lithography columns shape the electron beam by deflecting the electron beam across one or more shaping apertures. The resulting image in the shaped electron beam is then transferred to target 207 with a large total linear column demagnification. The requirement of large total linear demagnification (supplied by electron lenses 204 and 205) results in large column lengths, increasing electron-electron interactions that ultimately limit the electron current density of the column. The low electron current density results in a low throughput when the column is used in lithography.
Another major drawback in using known e-beam systems include the inability to modulate the electron beam without modulating the light source itself, usually a laser. Modulating a laser typically involves a large amount of control circuitry, requiring a large amount of space, and can be slow. In addition, in a patterned array of photocathodes, modulation of individual photocathodes in the array is extremely difficult. Finally, better resolution is required of lithography systems in order to meet future demands of semiconductor materials processing.
According to the present invention, a photocathode has a gate electrode that modulates and, in some embodiments, shapes the emission of an electron beam.
A photocathode emits electrons upon irradiation by a photon beam if the photon energy is greater than the work function of the photocathode. By masking the photocathode selectively with an opaque material, the emission is confined to pre-defined regions. Providing an electrically isolated gate structure that encompasses an emission region of the photocathode allows the intensity of the electron beam to be modulated by application of a gate bias voltage to the gate structure. If the gate structure has multiple segments, the electron beam emitted from the photocathode can also be shaped.
In a photocathode according to the present invention, an emission area is surrounded by a gate electrode that is offset from an electron emitting surface by an insulator. The gate electrode can be electrically controlled in order to turn the electron beam on or off or to vary the intensity of the electron beam. The electron beam is modulated in the region between the gate electrode and electron emitting surface rather than at a light beam source such as a laser, resulting in faster switching times and space savings in the electron beam system.
Embodiments of this invention can be utilized to form an array of photoemission sources each having a precisely controlled emitting region and position. In embodiments where the gate structure of each of the photoemission sources in the array includes a single gate electrode, each of the single gate electrodes in the array may be individually controlled or controlled in groups. In embodiments where the gate structure of each of the photoemission sources in the array includes multiple gate electrodes, each of the multiple gate electrodes may be individually controlled or controlled in groups. In yet other embodiments, the array of photoemission sources may include a combination of photoemission sources having a single gate electrode and photoemission sources having multiple gate electrodes where each gate electrode is individually or group controlled.
In general, emission regions can be of any size or shape that are within the limits of microfabrication technology. Some embodiments of the invention include self-biasing circuitry utilizing photoemission as the feed-back for stable emission intensity.
A photocathode includes a transparent substrate and a photoemission layer. The transparent substrate is transparent to a light source. The light source generates an array of light beams which are focused on an array of irradiation regions directly above the emitting areas on the photoemission layer. In one embodiment, the light source is a laser and the array of light beams results from the laser beam being split into multiple light beams using a beam splitter. Alternatively, the light source may be a UV lamp.
In some embodiments, each emitting area on the photoemission layer is a single pixel, a larger shape being formed by the aggregate of all of the pixels. Alternatively, the emitting area itself may represent any shape that is to be transferred to a target.
In some embodiments, masks are formed on top of the substrate in order to form the light beams into the desired images before the light beams are incident on the irradiation region. Other embodiments place a mask on the emitting surface of the photoemission layer. Yet other embodiments place a mask between the photoemission layer and the substrate in order to form the image in the light beam. In some embodiments, a back surface of the substrate, where the light beams are incident and opposite the photoemission layer, is shaped to provide lenses. The lenses help to focus the light beams onto the irradiation region.
According to the present invention, the emitting area is surrounded by an insulator. The emitting area itself is left uncovered by the insulator. In some embodiments, a single conductor is mounted on the insulator to form a gate electrode. In other embodiments, multiple electrically independent conductors are mounted around the emitting area on the surrounding insulator to form a gate electrode having multiple segments. Each segment of the gate electrode is independently controlled in order to turn on and off a corresponding portion of the electron beam that is initiated at the emitting area.
A photocathode according to the present invention is suitable for use in an arrayed electron source for conventional electron beam columns. Other embodiments of the invention are suitable for use as a miniaturized arrayed electron source for electron beam microcolumns. Some embodiments are suitable for use as a single gated source for conventional electron beam columns and microcolumns.
Photocathode arrays having gate electrodes with multiple segments allow variable shaping at the electron source in an electron beam lithography column without using shaping apertures or shaping optics. Use of these embodiments results in a shorter column length because of the reduced need for further beam shaping and demagnification. The shorter column length results in less electron-electron interactions and ultimately a higher throughput in systems such as lithography systems because of the higher intensity electron beams.
The invention and its various embodiments are further discussed along with the following figures and the accompanying text.